The present invention relates to the field of plasma control for the processing of a workpiece, such as a semiconductor wafer, in an inductively coupled plasma reactor.
In a plasma reactor, a gaseous plasma comprising ions is formed within the reactor chamber for etching from or depositing on the workpiece. Among others, plasma parameters such as ion density within the plasma and ion flux to the workpiece control the processing rate. Adjusting the plasma parameters affects the rate and quality of processing.
A significant limiting factor in workpiece processing is uneven charge build-up on the workpiece surface. Uneven charge build-up can cause a voltage differential to build on the workpiece. If the magnitude of the differential exceeds a breakdown voltage on the workpiece, an electrostatic discharge occurs, damaging the workpiece. To prevent damage, etch rates or etch profiles must be limited.
Uneven charge build-up occurs when electrons strike the surface of the workpiece with high lateral velocity. When electrons have high lateral velocity, they are unable to adequately penetrate deep vertical holes and trenches. Ions, however, do not attain such high lateral velocity, so they can be drawn by the sheath to the bottom of the holes and trenches.
In a plasma reactor, the gaseous plasma is formed by separating electrons from atoms or molecules to create ions. Electrons and ions have equal and opposite charges. The mass of the ions, however, is much greater than the mass of the electron. So, as the electron is separated from the molecule, it flies away with a much higher thermal velocity than does the ion. The thermal velocity of the electron or v.sub.the is at least 100 times greater than the thermal velocity of an ion or v.sub.thi. Electron and ion thermal velocities are given by the equations: EQU v.sub.the =(eT.sub.e /m.sub.e).sup.1/2
and; EQU v.sub.thi =(eT.sub.i /m.sub.i).sup.1/2
where
e=the unsigned charge of an electron; PA1 T.sub.e =temperature of the electron in volts; PA1 T.sub.i =temperature of the ion in volts; PA1 m.sub.e =mass of the electron; and PA1 m.sub.i =mass of the ion.
The lower thermal velocity ions, having a lower lateral velocity component, can be attracted by the voltage sheath to the surface of the workpiece so that they impinge upon the surface in a generally normal or perpendicular fashion. The direction of the electrons, however, are less affected by the sheath due to their high lateral component. High velocity electrons impinge the surface of the workpiece at acute angles far less than perpendicular. This leads to uneven charge build-up.
For example, uneven charge build-up results when a deep vertical hole is etched in the workpiece surface. Ions etching the bottom surface of the hole impart a positive charge, while high temperature less directionally controlled electrons are unable to penetrate the hole. The high temperature electrons, unable to penetrate to the bottom surface of the hole, strike adjacent surfaces on the wafer and cause negative charge build-up on those surfaces. When the potential between the surfaces reaches the break down voltage of the workpiece, an electrostatic discharge occurs damaging the workpiece. Reduction of component sizes has led to the need for more extreme workpiece surface profiles. As the depth increases and breadth decreases, damage due to uneven charge build-up is even more likely to occur and more difficult to control.
One way of reducing electron velocity would be to reduce source power. Unfortunately, this would also reduce plasma density. The ability to lower electron thermal velocity to reduce uneven charge build-up without sacrificing plasma density or ion flux would not only improve processing quality, it would also improve workpiece processing time. As plasma reactors typically process thousands of workpieces, a reduction in processing time translates into a significant reduction in reactor operating time. This would not only allow demand to be met more effectively, it would reduce materials expenses and reactor maintenance costs per workpiece. Plasma and process gases are typically housed within the chamber. Exposure to the plasma and other chamber gases can cause degradation of the chamber surfaces. As a result, reactors periodically must go down for cleaning or for part replacement. Limiting exposure of reactor components to plasma and other process gases reduces maintenance costs.
It is an object of the current invention to reduce workpiece processing time. It is another object of the invention to reduce electron temperature within a plasma without substantially affecting ion density or ion flux. It is a further object of the invention to provide an inductively coupled plasma reactor that reduces workpiece damage due to uneven charge build-up. It is yet another object of the invention to reduce degradation of reactor components.